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The Edge of the Universe

November 3, 2010

Since Hubble's time, distance in the universe has been measured by redshift. Hubble found that the universe is expanding, and the farther out you look, galaxies are receding from the Earth at greater speed. Relativity found a connection between the speed of a light source and the properties of its emitted light. Since the speed of light is a constant, something needs to change, and that something is the wavelength of emitted light. The light emitted by distant galaxies is shifted to longer wavelength. Since longer visible wavelengths are red, this is called redshift. The redshift is measured by a variable z, as follows:
z = (λobserved - λemitted)/λemitted

where λobserved is the observed wavelength of the light and λemitted is the emitted wavelength. Of course, all this depends on the idea that we know the emitted wavelength, but this is easily inferred from the pattern of spectral lines of hydrogen emission.

Observation of cosmologically distant objects is difficult in itself, but the added problem of slicing the light into small bands for spectral analysis makes measurement of z difficult for large values. Fortunately, telescope technology has advanced to the stage at which we now have the Hubble Space Telescope, and what more fitting instrument would there be to identify a candidate galaxy for a measurement that extends the limits of the redshift that Hubble discovered? This was demonstrated in a recent publication in Science of the observation of a galaxy with z = 8.6.[1-5]

Hubble deep field image showing galaxy at z=8.6

Hubble deep field image showing galaxy at z=8.6 (ESA/NASA)


This galaxy, called UDFy-38135539, was one of many galaxies identified by Hubble as having enhanced infrared emission indicative of a distant object.[5,6] The initial identification of the object was done by the Hubble Wide Field Camera 3, an infrared camera.[6] Spectroscopic measurement by the SINFONI spectrograph of the European Southern Observatory's Very Large Telescope in Chile demonstrated a z value of 8.5549 ± 0.0002. which corresponds to a distance of 13.12 billion light-years. This galaxy now holds the record for highest z value and, thereby, the greatest distance from Earth. This galaxy emitted this light when the universe was only 600 million years old, at a time when some of the hydrogen that permeates the universe was still unionized. After the first billion years, radiation stripped all the electrons from hydrogen, and the universe entered its present, transparent state. The previous record-holders were a galaxy at 12.93 billion light-years, and a stellar event known as a gamma-ray burster at 13.09 billion light-years. Actually, no object was seen in the later, just the emitted gamma-ray burst.

Hubble's replacement, the James Webb Space Telescope, which is scheduled to be launched in 2014, will be able to look back to a time at which the universe was just 300 million years old. Galaxies this old will be hard to find, even for the Webb, since few galaxies existed in those early years, and they are expected to be dimmer. The spectrograph of UDFy-38135539 itself required a sixteen hour exposure, and it's conjectured that it was only because of hydrogen ionization by some unseen companion galaxies that it was as bright as it was. Total ionization of hydrogen did not occur until the z ≈ 6 epoch.

These measurements are not easy. There was an earlier report of an object with a z value of ten, but further work confirmed neither the object nor the redshift.[2] Astronomers now discount this prior claim. The discovery of UDFy-38135539 by Hubble was possible only because of a camera upgrade. The Hubble Wide Field Camera 3 has forty times the sensitivity of the camera it replaced.

While on the topic of the edge of the universe, let me mention a nice science fiction fansite, The Website at the End of the Universe. Each year, they publish a free online calendar featuring vintage cover art from pulp magazines.

IR Image of Messier 81

One of my favorite galaxies, Messier 81, also called M81. This infrared image was taken by the Spitzer Space Telescope (NASA).


References:

  1. M. D. Lehnert, N. P. H. Nesvadba, J.-G. Cuby, A. M. Swinbank, S. Morris, B. Clément, C. J. Evans, M. N. Bremer and S. Basa, "Spectroscopic confirmation of a galaxy at redshift z = 8.6," Nature, vol. 467, no. 7318 (October 21, 2010), pp. 940-942.
  2. Douglas Pierce-Price, "Clearing the Cosmic Fog-The Most Distant Galaxy Ever Measured," European Southern Observatory Press Release eso1041 (October 20, 2010).
  3. Andrew Fazekas, "Universe's Most Distant Object Spotted," National Geographic, October 20, 2010.
  4. Ron Cowen, "New cosmic distance record-holder," Science News, October 20, 2010.
  5. Hubble finds most distant primeval galaxies, ESA/Hubble Science Release heic1001, January 5, 2010.
  6. Hubble Reaches the "Undiscovered Country" of Primeval Galaxies, Space Telescope Science Institute News Release Number STScI-2010-02, January 5, 2010.

Permanent Link to this article

Linked Keywords: Edwin Hubble; universe; redshift; expanding; speed of light; wavelength; galaxies; hydrogen emission; cosmology; telescope; Hubble Space Telescope; Science; infrared; Hubble Wide Field Camera 3; SINFONI spectrograph; European Southern Observatory; Very Large Telescope; light-year; The Website at the End of the Universe; Messier 81; Spitzer Space Telescope.

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